The invention relates to a method for the manufacture of ultrasonic transducers, with at least one ultrasonic transducer configuration whose active surface exhibits recesses and whose sound radiation field is to be prescribable in terms of focal distance, focal width and angle of aperture in connection with the focus and ultrasonic frequency.
In ultrasonic technology, particularly in the electromedical field or in the field of material testing, there is a desire for optimum adaptation of the sound radiation field of the respective ultrasonic transducer to the conditions. Thus, the focal distance, for example, should always be optimally adjustable to the respective distance of the examination point from the transducer surface. The focal width and the angle of aperture of the sound radiation field should likewise be as small as possible so that the lateral resolution becomes optimum. The influence of side lobes of the sound radiation field should be as low as possible. Finally, all of these conditions should be given for any desired use purpose, thus, for example, for ultrasonic devices with or without a preliminary water segment or some other format. Known methods for the manufacture of ultrasonic transducers do not allow up to now of a clear, optimum matching between focal interval, focal width and angle of aperture of the sound radiation field, as this is actually desired. Thus, the periodical "Journal of Acoustical Society of America", Vol. 44, No. 5, 1968 describes such a manufacturing method on pages 1310 through 1318, particularly, however, on page 1312, in which the individual values cannot be prescribed and varied separately from one another. The ultrasonic transducers manufactured according to the method, moreover, are only laid out for continuous sound not, however, also for pulse-echo operation. Upon employment in pulse-echo operation, there automatically and inevitably ensue greater values for the focal distance and focal width and also the angle of aperture, which is actually undesired. In ultrasonic transducers, the lateral resolution can be improved by setting the focus and angle of aperture of the sound radiation field behind the focus to be as small as possible. In ultrasonic transducers with or without a preliminary water segment, as is known, a smaller angle of aperture can be achieved by means of enlarging the active transducer surface or by means of prescribing a weaker radius of curvature of the artificial focusing (mechanical or electronic) of the transducer. An increased active transducer surface or a weaker radius of curvature, however, always lead to an expansion of the sound radiation field at the focus and behind it. The result is a deterioration of the lateral resolution both at the focus as well as in the examination area of the sound field lying behind it. If one eliminates these measures, i.e., thus, the transducer surface is reduced or is more strongly, artificially focused, then the lateral resolution directly at the focus is improved. However, directly behind the focus the lateral resolution deteriorates quickly since the angle of aperture is now significantly greater than before. This is also true in this sense for the proposals for the manufacture of strongly focused transducers made in the essay "A real-time B-scanner with improved lateral resolution" by C. B. Burckhardt et al. Thus, for example, the spherically curved transducer illustrated in FIG. 1 on page 81 is geometrically strongly focused so that for different dB lobes focuses lying close to one another and, thus, good lateral resolution in this region, ensue. Behind this focal range, however, there appears a very large angle of aperture which immediately and quickly again deteriorates the lateral resolution. Thus, the area of good lateral resolution is limited in an undesirable manner to an examination area which is much too small. The ultrasonic transducer illustrated in FIG. 2 on page 82 of the essay is constructed annularly, whereby, according to FIG. 4 on page 84, the ring consists of individual partial elements which follow one another. The annular form of the transducer favors the formation of interference structures. By so doing, relatively strong focusing and a small angle of aperture and, thus, good lateral resolution, too, ensue. However, the increased presence of interference structures is accompanied by an increase of the influence of side lobes. These side lobes lead to image falsifications.